The present invention generally relates to optical information storage units, and more particularly to an optical information storage unit which is capable of adjusting a focal offset and is suited for recording and reproducing a magneto-optic signal on and from a recording medium.
In this specification, an "information storage unit" refers to an apparatus which records information on and/or reproduces information from a recording medium.
In an optical information storage unit, a bundle of rays emitted from a light source is reflected by a polarization beam splitter and is irradiated on a recording surface of an optical recording medium. The bundle of rays reflected by the recording surface of the optical recording medium passes through the polarization beam splitter and is directed towards a photodetector. When the reflected bundle of rays from the optical recording medium passes through the polarization beam splitter, astigmatism is generated, and a focal error signal can be generated by use of this astigmatism. Conventionally, a cube-shaped polarization beam splitter is generally used as the polarization beam splitter.
FIG. 1 is a diagram showing an important part of an example of a conventional optical information storage unit. In FIG. 1, a bundle of rays emitted from a light source 14 is converted into parallel rays by a collimator lens 15, and supplied to a cube-shaped polarization beam splitter 16 as a P-polarized light. The bundle of rays supplied to the polarization beam splitter 16 is transmitted through and is reflected by the polarization beam splitter 16 depending on a polarization characteristic of the polarization beam splitter 16.
The bundle of rays transmitted through the polarization beam splitter 16 is converged to a diffraction limit by an objective lens 17, and is irradiated on a recording surface of a magneto-optic recording medium 18. The bundle of rays reflected by the recording surface of the magneto-optic recording medium 18 again passes through the objective lens 17, is reflected by the polarization beam splitter 16, and is then supplied to a convergent lens 19. The convergent lens 19 converts the bundle of rays reflected by the polarization beam splitter 16 into a convergent light, and supplies the convergent light to a cylindrical lens 20. Astigmatism is generated as the convergent light passes through this cylindrical lens 20. The astigmatism which is generated is used to generate a focal error signal. The convergent lens 19 has an adjustable structure for enabling adjustment of an offset of the focal error signal, and the convergent lens 19 is adjustable in a direction of an optical axis along which the bundle of rays travel.
The bundle of rays passed through the cylindrical lens 20 further passes through a Wollaston prism 21 where the bundle of rays is separated into 2 polarized light components having electrical vectors in mutually perpendicular directions and 1 polarized light component in which the 2 polarized light components coexist. The total of 3 polarized light components obtained from the Wollaston prism 21 are supplied to a photodetector 22.
FIG. 2 is a plan view showing the construction of the photodetector 22. The photodetector 22 includes a 4-part detector 22a, a detector 22b, and a detector 22c. The 2 polarized light components having the electrical vectors in mutually perpendicular directions are respectively supplied to the detectors 22b and 22c and are subjected to a photoelectric conversion. Output detection signals of these detectors 22b and 22c are converted into a magneto-optic signal via a differential amplifier 230. On the other hand, the 1 polarized light component in which the 2 polarized light components coexist is supplied to the 4-part detector 22a and is subjected to a photoelectric conversion. The focal error signal is generated based on output detection signals of 4 detector parts of the 4-part detector 22a.
The conventional optical information storage unit uses an expensive cube-shaped polarization beam splitter as the polarization beam splitter 16. Furthermore, the bundle of rays emitted from the light source 14 is supplied to the polarization beam splitter 16 after being converted into the parallel rays by the collimator lens 15. For this reason, the bundle of rays remains in the state of the parallel rays even when being directed towards an optical system which detects the magneto-optic signal, and the convergent lens 19 is inevitably required in order to generate the focal error signal.
In order to enable omission of the convergent lens 19, it is conceivable to supply a divergent bundle of rays emitted from the light source 14 as it is to the polarization beam splitter 16 and thereafter convert the divergent bundle of rays into the parallel rays. But in order to obtain a laser power that is required to carry out the recording using this conceivable method, a numerical aperture of the collimator lens 15 cannot be made small, and as a result, a focal distance of the collimator lens 15 becomes restricted. Therefore, it is necessary to provide a concave lens in order to obtain a sufficiently high focal error detection sensitivity, and it is difficult to reduce the number of optical parts.
On the other hand, it is also conceivable to use as the polarization beam splitter 16 an inexpensive plate-shaped polarization beam splitter in place of the expensive cube-shaped polarization beam splitter. However, the focal distance of the collimator lens 15 becomes restricted in this case, and it is difficult to secure a sufficiently large space for arranging the plate-shaped polarization beam splitter between the light source 14 and the collimator lens 15.
In addition, some kind of means must be provided to correct an offset of the focal error signal, but the conventional optical information storage unit adjusts the offset by moving a concave lens or a convex lens in a direction along the optical axis. Consequently, when a positional error of an optical element occurs due to a change in the environment such as a temperature change, there was a problem in that it is impossible to generate a stable focal error signal.